1. Field of Invention
The present invention relates to an overdrive system and a method of operating the overdrive system of a display device. More particularly, the present invention relates to an overdrive system and a method of operating the overdrive system of a liquid crystal display (LCD) device.
2. Description of Related Art
A liquid crystal display (LCD) device applies electric field to drive liquid crystal molecules from an initial molecular alignment state to a different molecular alignment state. The change in molecular alignment brings about a change in optical property and visual appearance. In general, an LCD device may operate at a low operating voltage and consume very little electric power. Moreover, the LCD device can easily be driven by large scale integrated (LSI) circuits.
Intrinsic properties of liquid crystal molecules permit the application of an external electric field (a voltage) to re-orient their molecular alignment. Through selective alignment of molecules, the transparency of a liquid crystal pixels is changed to form an image pattern on the LCD. However, liquid crystal molecules have a relative slow response to electric field. For example, the application of a data voltage such as 5V to an image pixel may not rotate the liquid crystal molecules to a destined angle within a preset time period.
Slow response to data voltage compared with a conventional cathode ray tube (CRT) display means that blurred images may form when motion pictures are displayed. In other words, the transmission rate of image data to the LCD is so much faster than the response time of the LCD device that the liquid crystal molecules within the LCD device fail to follow well. To boost the response, some manufacturers have developed an overdrive circuit.
In general, to rotate liquid crystals molecules inside the pixel cell to a destined orientation at 5V within a preset time period, a data voltage higher than 5V needs to be applied. That is a larger data voltage applied to the pixel cell increases the rotation rate of the liquid crystal molecules. For example, to rotate liquid crystal molecules to an angular orientation θ1 that corresponds to the application of 5V within a preset time period T, a larger data voltage such as 6V is applied instead. Although the liquid crystal molecules are unable to rotate to an angular orientation θ2 (θ2>θ1) that corresponds to the application of a 6V within the time period T, the higher voltage permits the rotation of the liquid crystal molecules to an angle θ1 that corresponds to the application of 5V within the same time period T. The concept of designing an overdrive circuit is based on this fact.
To deploy the aforementioned type of overdrive circuit, the overdrive voltage must be computed based on a previous data frame. For example, if the previous state corresponds to a 0V data voltage and the next desired state is a 5V state, a higher data voltage such as 6V may be applied to a liquid crystal cell. However, if the previous state corresponds to a 5V data voltage and the next desired state is still a 5V state, an identical data voltage, in other words, 5V may be applied to the liquid crystal cell. On the other hand, if the previous state corresponds to a 3V data voltage and the next desired state is a 5V state, a moderately high data voltage such as 5.5V instead of a full 6V may be applied to the liquid crystal cell.
FIG. 1 is a block diagram showing the overdrive function of a conventional display device. A computer terminal 10 and a display terminal 20 are shown in FIG. 1. The display terminal 20 is a liquid crystal device (LCD) while the computer terminal 10 is controlled by an operating system 18. The computer terminal 10 communicates with the display terminal 20 via a display interface. Through the operating system, data to be displayed is transmitted to a display interface. The display interface re-transmits the data to the display terminal 20 and forms an image on a screen. The display interface further include a VGA BIOS 12, VGA chip 14 and video RAM (VRAM) 16.
The VGA chip 14 outputs image data to be displayed to a signal converter 21 of the display terminal 20. From the signal converter 21, the signals are transmitted to a timing controller 22. The timing controller 22 reads out the display data of the previous frame from a frame buffer 23. Accordingly, suitable overdrive display data is read out from a overdrive look-up table 24. The overdrive display data are transmitted to a driver 25 for driving a liquid crystal panel 26.
Although the aforementioned system is able to overdrive the LCD device, the display terminal 20 has a complicated structure and a high cost of production. If the frame buffer 23 has a resolution of about 1024 bits×768 bits, uses three primary color (RGB) and a 6-bit display, then the frame buffer 23 requires 1024×768×3×6 bits or about 1.73 MB. In other words, the display terminal 20 needs a memory having a memory capacity of at least 1.73 MB just to hold the frame data. In addition, the overdrive look-up table 24 needs a ROM having a memory capacity of at least 64×64×3 bytes for holding overdrive display data values and hence incurs additional production cost.
Furthermore, since additional leads are required to facilitate the communication and control between the frame buffer 23 and the timing controller 22, size of the timing controller 22 is increased.